The present invention relates to a thermosensitive inked element for non-impact printers of thermal type, comprising a base support of plastics material coated on one side with a thin layer of a mixture transferable, when it is subjected to heat and pressure, to a document being printed. More particularly, the black or coloured inked element, normally a ribbon, is adapted to be disposed between a thermal head and a sheet of ordinary paper and to transfer the heated portions of the inking layer to this paper.
Normally, the inking layer comprises a mixture of solid ink and a binder with a relatively low melting point, whereby this layer can be melted or softened and transferred by means of simultaneous application of heat and pressure. The quality of the printing obviously depends on the speed of the printer, that is on the duration of the energization of a point of the printing head on a corresponding portion of the ribbon.
The quality of the printing moreover depends on the composition of the inking layer, which must soften or melt in the least time possible for taking variations in printing speed and must adhere cleanly to the paper.
An inked ribbon for a thermal printer has been proposed wherein the inking layer comprises as binder a thermoplastic resin adapted to melt at a temperature between 80.degree. C. and 100.degree. C. Since this resin is relatively rigid and brittle at room temperature, in order to reduce the tendency to crack it is spread in a layer with a thickness of the order of tens of microns. This ribbon is not suitable for high-speed printers, both because of the thickness of the ink and because of the high melting temperature.
A thermal ribbon has also been proposed wherein the mixture of the inking layer comprises, in addition to the binding resin, a hydrocarbon wax or plasticizing resin which lowers the melting point of the layer and allows a layer thickness of the order of 2 to 4.mu.. This layer adheres tenaciously to the paper without being absorbed by it and is not subject to blurring when it is rubbed. However, this ribbon requires a relatively high pressure and adheres with difficulty in the case of high-speed printers.
A thermal ribbon has also been proposed wherein the mixture of the inking layer is very thin and comprises a mixture of natural wax having a relatively high melting point with an esterified wax having a relatively low melting point, in the ratio of 2 parts of esterified wax to 1 part of natural wax. To this there is added a predetermined amount of oil for making the ink penetrate into the paper. Although this layer shows a good readiness to melt, it nevertheless has the disadvantage of permeating the fibres of ordinary paper, as a result of which it tends to form more or less pronounced runs according to the characteristics of the paper. Moreover, it is liable to spread and blur when the printed symbol is rubbed, for example with a finger.